REVIEW

Malignant Transformation and Associated Biomarkersof Ovarian Endometriosis: A Narrative Review

Liudmila M. Mikhaleva . Aleksandr I. Davydov . Olga I. Patsap .

Elizaveta V. Mikhaylenko . Vladimir N. Nikolenko . Margarita E. Neganova .

Sergey G. Klochkov . Siva G. Somasundaram . Cecil E. Kirkland .

Gjumrakch Aliev

Received: March 13, 2020 / Published online: May 8, 2020� The Author(s) 2020

ABSTRACT

This review focuses on pathogenesis ofendometriosis, its possible biomarkers and rolein endometriosis-associated ovarian cancer. Weanalyzed various databases to obtain newinsights, theories, and biomarkers associatedwith endometriosis. There are several theories ofendometriosis development and biomarker

changes including atypical forms. A number ofstudies have attempted to establish specific,reliable biomarkers to help diagnoseendometriosis and endometriosis-associateddiseases on the basis of different pathogeneticpathways. Nevertheless, despite intensiveresearch extending even to the molecular level,the origin, natural history, malignant transfor-mation, and laboratory management ofendometriosis and related diseases are not yetclearly defined. Therefore, early laboratory diag-noses of endometriosis, its atypical form, and

Digital Features To view digital features for this articlego to https://doi.org/10.6084/m9.figshare.12179421.

L. M. Mikhaleva � G. Aliev (&)Department of Clinical Pathology, Federal StateBudgetary Institution ‘‘Research Institute of HumanMorphology’’, 3, Tsyurupy Str, Moscow 117418,Russian Federatione-mail: cobalt55@gallyinternational.com;aliev03@gmail.com

A. I. DavydovDepartment of Obstetrics, Gynecology andPerinatology, Institute of Clinical Medicine, I.M.Sechenov First Moscow State Medical University(Sechenov University), 8/2, Trubetskaya Str.,Moscow 119991, Russian Federation

A. I. Davydov � O. I. PatsapDepartment of Pathology, City Clinical HospitalAfter Named S.S. Udina, 4, Bld., 3, KolomenskyPassage, Moscow 115446, Russian Federation

E. V. Mikhaylenko � V. N. Nikolenko � G. AlievI. M. Sechenov First Moscow State MedicalUniversity of the Ministry of Health of the RussianFederation (Sechenov University), 8/2 TrubetskayaStr, Moscow 119991, Russia

V. N. NikolenkoDepartment of Normal and Topographic Anatomy,M.V. Lomonosov Moscow State University,Leninskie Gory, 1, Moscow 119991, Russia

M. E. Neganova � S. G. Klochkov � G. AlievInstitute of Physiologically Active Compounds,Russian Academy of Sciences, 1 Severny pr,Chernogolovka, Moscow Region 142432, Russia

S. G. Somasundaram � C. E. KirklandDepartment of Biological Sciences, SalemUniversity, Salem, WV 26426, USA

G. AlievGALLY International Research Institute, 7733 LouisPasteur Drive, #330, San Antonio, TX 78229, USA

Adv Ther (2020) 37:2580–2603

https://doi.org/10.1007/s12325-020-01363-5

endometriosis-associated ovarian tumors areimportant problems that require further study inthe context of advanced therapeutic strategies toprovide maximal health benefits to patients.

Keywords: Biomarkers; Endometriosis-associated tumors; Endometriosispathogenesis; Prognosis of endometriosis;Treatment of endometriosis; Women’s health

Key Summary Points

Ovarian endometriosis is one of theleading causes of female infertility and is aknown risk factor for the development ofovarian cancer.

The review summarizes data in currentliterature evidence about ovarianendometriosis, its pathogenesis,associated and probable biomarkers, casesof malignant transformation, principles ofdiagnosis and treatment.

Despite intensive research ofendometriosis even at the molecular level,its origin, natural history, malignanttransformation, and laboratorymanagement are not yet clearlyinvestigated.

Early laboratory diagnoses ofendometriosis, its atypical form, andpossibility of transformation toendometriosis-associated ovarian cancersare important problems that requirefurther study.

INTRODUCTION

Endometriosis is an important medical andsocial problem. It is the third most commonfactor in the pathogenesis of gynecologic dis-eases after inflammatory processes and uterineleiomyoma. Previous studies have shown thatits incidence in fertile women ranges from 10%to 50%. It is one of the most frequent causes ofreproductive malfunction, concomitant

infertility, and chronic pelvic pain syndromedevelopment [1]. About 80% of women suffer-ing from chronic pelvic pain and up to 50% ofwomen with diverse forms of infertility havebeen shown to have endometriosis. Ovarianendometriosis and adenomyosis can result ininfertility and is second in frequency afterinflammatory processes of the female genitaltract, making up from 37% to 50% of suchobservations [1]. Endometriosis is an estrogen-dependent gynecologic disease, and it rarelyappears before menarche or after menopause.The stabilization or regression of endometriosiscould be observed during pregnancy or medicaltreatment for amenorrhea [1]. The classical roleof estrogen and the great importance of para-crine and autocrine mechanisms are wellestablished [1–4]. Importantly, endometriosis isa known risk factor for ovarian cancer [5] andcan transform to atypical form and evenundergo malignant transformation in 0.7–2.5%of cases [6, 7]. All electronic searches wereconducted on four different electronic data-bases (MEDLINE, Scopus, Embase, and PubMed)to retrieve relevant studies conducted over thelast 30 years. The search terms used were asfollows: ‘‘endometriosis,’’ ‘‘biomarkers’’, ‘‘en-dometriosis pathogenesis’’, ‘‘endometriosis-as-sociated tumors’’, ‘‘treatment of endometriosis’’.

The aim of this review is to summarize theinformation in modern literature about the com-plexityofpathogenesisof endometriosis, its abilityto undergo malignant transformation, possiblebiomarkers that can be used in early diagnostics,andprinciplesof treatment. This article isbasedonpreviously conducted studies and does not con-tain any studies with human participants or ani-mals performed by any of the authors.

COMPLEXITYOF THE PATHOGENESISOF ENDOMETRIOSIS

More than ten theories on the origin ofendometriosis have been proposed, each ofwhich offers an explanation of its etiology,pathogenesis, and pathological process; but noone theory explains all of the various forms andmanifestations of this disease [1, 2, 8–10]. These

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theories include diverse factors such as retro-grade menstruation, altered immune response,coelomic metaplasia, embryonic rest theory,lymphovascular metastasis, molecular alter-ations, genetic instability, etc. (Figs. 1 and 2).

The most quoted two theories of endome-trial implantation assume either that endome-trial cells reflux or that retrograde menstruationinvolving the fallopian tubes results inimplantation and proliferation elsewhere [6–8].While both theories are plausible, they haveweaknesses. The implantation theory cannotexplain the development of endometriosis in

10–15% of women. The reflux theory ofendometrial tissue dispersing through the fal-lopian tubes during menstrual periods is analmost universal phenomenon, but it fails toexplain the admittedly rare cases ofendometriosis when the menstrual uterus isabsent [8–10].

According to the coelomic metaplasia the-ory, endometriosis arises from mesothelial cellson the ovarian surfaces that are transformedinto endometriotic gland cells. This theorysupports the rare cases of endometriosis amongmen and pubertal girls [11].

Fig. 1 Possible molecular mechanisms in endometrioticcells proliferation, survival, and progression towardsmalignancy. BAF BRM-associated factor, ARID1a/b AT-rich interactive domain-containing protein 1A/B, SMARscaffold/matrix attachment region-binding protein 1,DPF1-3 zinc and double PHD fingers family 1 protein,ACTL6a/b actin-like protein 6A/B, GPCR G-protein-coupled receptors, GRB2 growth factor receptor-boundprotein 2, RAS protein superfamily of small GTPases, Raf-1 proto-oncogene serine/threonine-protein kinase, TNFtumor necrosis factor, PI3K phosphoinositide 3-kinases,TAK1 mitogen-activated protein kinase kinase kinase 7,MAPK3 and ERK1/2 mitogen-activated protein kinase 3/2, MKK4/7 dual-specificity mitogen-activated proteinkinase kinase 4/7, JIP1/2/3 JNK-interacting protein 1/2/3, JNKs(1–3) c-Jun N-terminal kinases (1–3), ATF2activating transcription factor 2, p53 tumor protein p53,STAT3 signal transducer and activator of transcription 3,

BAX Bcl-2-like protein 4, Bcl-2 B cell lymphoma 2, AKTprotein kinase B, FOXO3 forkhead box O3, MEK1/2mitogen-activated protein kinase kinase 1/2, ELK1 ETSlike-1 protein Elk-1, ESR1 estrogen receptor 1, MITFmicrophthalmia-associated transcription factor, PAX6paired box protein, cFos proto-oncogene, member of abigger Fos family of transcription factors, ETS erythroblasttransformation specific member, MYC MYC proto-onco-gene, VEGF vascular endothelial growth factor, MNKmitogen-activated kinase, mTORC1/2 mammalian targetof rapamycin complex 1/2, p7056K ribosomal protein S6kinase beta-1, CAD carbamoyl-phosphate synthetase 2,SGK1 serum and glucocorticoid-regulated kinase 1, PKCprotein kinase C, PTEN phosphatase and tensin homolog,PIP 2/3 phosphatidylinositol 4,5-bisphosphate/trisphos-phate, PDK1 pyruvate dehydrogenase lipoamide kinaseisozyme 1

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Embryonic rest theory proposes that someendometrial cells may develop in the abdomencavity during embryogenesis [12]. The fetalsystem controls and directs embryogenesis, butthe mechanism is not fully understood.Abnormalities of this control system may giverise to detectable abnormalities of the adultimmune system [13]. In turn, these abnormali-ties may control the degree of ‘‘aggressiveness’’of the endometriosis and may result in differentclinical behavior of endometriosis [14, 15].

There is a new unifying theory thatendometriosis is the result of endometrial stemcells that migrate and proliferate duringembryogenesis [16]. This theory is based onfinding small focuses with glands and stroma infemale fetuses in the cul-de-sac of the peri-toneum that are supposed to be the remnants orthe result of metaplasia of the Mullerian system.These remnants may contain endometrial stemcells that can produce endometriotic foci [16].Interestingly, endometrial stem cells differ frombone marrow stem cells by overexpression ofimmune-function related gene pathways [16].

DEVELOPMENTAL FACTORS

Endometriosis means the presence of ectopictissue that is similar to an endometrium withglands and stroma. The endometria of womenwith endometriosis may have abnormal featuresin comparison to the endometria of healthy

women [8, 9]. The abnormal endometrium maybe able to protect itself from destruction by theimmune system by expressing specific antigensthrough the accumulation of various popula-tions of immune cells, as well as synthesis andsecretion of immunosuppressive factors[8, 9, 17]. There are several characteristic fea-tures of the endometrium in the case ofendometriosis, including:

(a) Production of its own estrogen in a largeamount [8, 9, 15].

(b) Implantation into the peritoneum[15, 17, 18].

(c) Proliferation and invasion into surround-ing tissues [15, 17, 18].

(d) Aggressive growth and adhesion in theperitoneum [15, 17, 18].

(e) Self-defense from physiological apoptosis[15, 17, 18].

(f) Pathological expression of heat shock pro-teins and excessive angiogenesis[4, 8, 9, 17, 18].

Additional theories propose that theimmune system must be compromised to allowthe ectopic endometriotic tissue to grow [13].Endometriosis is associated with inflammation,so macrophages play a key role in its develop-ment: they are found in endometrioma walls,peritoneal fluids, and other ectopic foci. Mac-rophages secrete many products such as tumorgrowth factor-b (TGFb), vascular endothelialgrowth factor (VEGF), interleukin-1 (IL-1),

Fig. 2 Pathogenesis and progression of endometriosis. OV ovary, FT fallopian tube, RM retrograde menstruation, CMcoelomic metaplasia, ETi endometriotic tissue, Fe iron, Mph macrophage, M monocytes, L lymphocytes

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prostaglandin E2 (PGE2), and macrophagemigration inhibitory factor (MIF). Activatedmacrophages can enhance oxidative stressconditions via the production of lipid peroxidesand other by-products of the reaction betweenapolipoproteins and peroxides [19]. The resultof these events is the accretion of the pro-in-flammatory mediators, leading to inflammationin women with endometriosis [19]. TGFb playsa significant role in increasing the rate of post-surgical adhesion formation [20]. Experimentalmurine models show that macrophage activa-tion after endometrial tissue implantationincreases secretion of VEGF [21]. IL-1 is a mac-rophage-derived cytokine inducing the expres-sion of cyclooxygenase-2 (COX-2) andinterleukin-8 (IL-8) to facilitate migration, pro-liferation, and angiogenesis in endometriotictissue [22]. PGE2 affects the leukocyte popula-tion and promotes angiogenesis via its effect onestrogen and VEGF upregulation [23]. It inhibitsapoptosis and upregulates fibroblast growthfactor 9 (FGF9) to promote cell proliferation aswell [13].

Endometriotic lesion cells show increasedadhesive capability to extracellular matrixcomponents, such as collagen type IV, laminin,vitronectin, and fibronectin [20, 24].Endometriosis is associated with aberrantexpression of E-cadherin, b-catenin, and inte-grins. Integrins are a group of cell-to-cell adhe-sion molecules and enforce cell attachment tothe extracellular matrix proteins, thereby pro-viding cell migration and invasiveness [25]. Ofnote, b-catenin is an important molecule in cell-to-cell adhesion and intracellular signaling thatbinds to intracellular E-cadherin and connectsE-cadherin to the cell cytoskeleton [26]. TheE-cadherin–b-catenin complex plays a crucialrole in epithelial cell–cell adhesion and in thetissue architecture [27]. Cancer initiation andprogression affect aberrant expression of cad-herins and integrins [28]. Controversial reportshave been published describing the expressionlevels of these adhesion proteins in the case ofendometriosis. In keeping with this, b-cateninexpression may be involved in the pathogenesisof endometriosis via increasing expression of b-catenin and activation of the Wingless Int-1(Wnt)/b-catenin complex [27, 29, 30].

Endometriosis showed decreasing b-cateninexpression in comparison to ovarianendometrioid carcinoma. This fact reflects thatthe pathogeneses of endometriosis andendometrioid carcinoma have different alter-ations in the E-cadherin–b-catenin complex[13]. Thereby different epithelial adhesionmolecules are involved in the initiation andprogression of benign endometrioid lesions[25]. Wnt/b-catenin complex plays a pivotalrole in stem cell regulation and cell develop-ment by merging signals from other pathways,such as TGFb and FGF. This complex also targetscell migration and proliferation genes [31].TGFb provides migration and proliferation offibroblasts in ovarian endometriotic lesions[29]. Loss of E-cadherin expression may beconnected to the aggressive behavior and inva-sive growth of peritoneal endometriotic lesions[27, 32]. E-cadherin expression in endometrialcells has been reported to be constantthroughout the menstrual cycle [32, 33]. How-ever, another study found that E-cadherinmRNA was significantly lower in the prolifera-tive phase than in the secretory phase [34].Thus, E-cadherin expression may be dependenton the phase of menstrual cycle and stage ofendometriosis. Moreover, E-cadherin expres-sion patterns in endometriotic tissues are con-tradictory; therefore, the role of E-cadherin inthe development and progression ofendometriosis remains unclear [13].

MicroRNAs (miRNAs) are small non-codingRNA molecules containing about 22 nucleotidesthat are found in plants, animals, and someviruses and provide post-transcriptional regula-tion of gene expression [35]. miRNAs take partin several biological processes including cellproliferation, differentiation, and apoptosis.Epigenetic regulation by miRNAs plays animportant role in endometriosis developmentsupported by the fact of differential expressionof miRNAs in endometriosis tissues comparedto normal endometrium [36–38]. Target generecognition can be complicated by singlenucleotide polymorphisms within miRNAbinding sites. This means that single nucleotidepolymorphisms within or near a miRNA targetsite may serve as an epigenetic modulator ofendometriosis predisposition and may explain

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familial and twin cases that suggestendometriosis is inherited in a polygenic man-ner [39–41]. However, these studies have shownlimited consistency or conflicting results, andno miRNA-based diagnostic test has been vali-dated in an independent patient cohort [42].

Bone morphogenetic protein receptor type-1B (BMPR1B) is a transmembrane receptormediating TGFb signal transduction and hasbeen shown to be a tumor suppressor in ovariancancer. Endometriosis and ovarian cancer gly-cosylation-associated biomarker CA125 (carbo-hydrate antigen 125) inversely correlated withBMPR1B in endometrial cells. Polymorphismat BMPR1B 30UTR in the miR-125b binding siteviolates its affinity toward the miRNA, whichmay result in insufficient post-transcriptionalrepression. Genetic variations in the mir-125bseed region reduces suppressive effect of mir-125b and result in upregulation of BMPR1B.Elevated BMPR1B levels in endometrial cellshave been shown to reduce cell proliferationand migration activity by downregulation of IL-1b, reflecting a lower risk for endometriosis [43].

BIOMARKERS ASSOCIATEDWITH ENDOMETRIOSIS

A study by Gupta et al. [44] with 2729 partici-pants studied the proteome, finding that 17b-hydroxysteroid dehydrogenase 2 (17bHSD2), IL-1R2, caldesmon, and other neural markers (va-soactive intestinalpeptide (VIP), calcitoningene-related peptide (CGRP), substance P (SP), neu-ropeptide Y (NPY), and a combination of VIP,PGP 9.5, and SP) have the promising ability todiagnose endometriosis accurately. Laparoscopyhas been the gold standard for diagnosis ofendometriosis and, today, non-invasive meth-ods may be used only for research purposes[44, 45].Nevertheless, itwas shown that there is acorrelations between CA125 and carbohydrateantigen 19-9 (CA19-9) and dehydroepiandros-terone sulfate (DHEA-S) (R = 0.52 and R = 0.49).This initial result reflects that androgen-depen-dent increases in CA125 and CA19-9 levels arepotentially significant diagnostic biomarkers ofendometrial pathology [46].

The diagnosis of peritoneal endometriosisrelies on a visual inspection using laparoscopyfollowed by histological confirmation [47].Great efforts have been made to find non- orsemi-invasive tests for diagnosis ofendometriosis. The most important goal ofthese tests is to identify 100% of woman withendometriosis or other significant pelvicpathology who could form clinical groups thatmight benefit from surgery for endometriosis-associated pain or infertility [48–50]. A non-in-vasive diagnostic test could be developed forserum or plasma, urine, endometrial fluid, ormenstrual fluid that can be recovered from theposterior vaginal fornix and from the cervixduring speculum examination. A semi-invasivetest could be developed using peritoneal fluidobtained after transvaginal ultrasound-guidedaspiration or in the endometrium obtained aftertranscervical endometrial biopsy [48, 49]. Basedon these findings, there is a need to develop asensitive diagnostic test for endometriosisbecause of the unavailability of a current validtest [48, 51, 52].

There is consensus in the WorldEndometriosis Society that the development ofa reliable, non-invasive test is one of the topresearch priorities in endometriosis [53, 54].The development of such a test extending frominitial biomarker discovery to a clinicallyapproved biomarker assay is a difficult, time-consuming process [52, 54].

Overall, most studies of endometriosisbiomarkers have remained at the level ofexploratory preclinical studies aimed to identifypotential biomarkers [51]. Only a few effectivelypassed the preclinical development and valida-tion of a clinically useful non-invasive diag-nostic test. A clinically reliable test forendometriosis is expected to have a profoundimpact on reduction of health care and indi-vidual costs by reducing expensive hit-and-misstreatments [49, 55]. Taken together, a non- orsemi-invasive test would not only reduce thecosts associated with endometriosis diagnosisbut also improve the quality of life in womenwith endometriosis by enabling early diagnosisand treatment.

Blood is an important source of biomarkersbecause it allows repeated measurements, is

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easily obtained, and is highly suitable for high-throughput measurements [56]. Endometriosisbiomarkers are mainly glycoproteins, growth oradhesion factors, hormones, or proteins relatedto immunology or angiogenesis [48, 57, 58].Unfortunately, neither a single biomarker nor apanel of biomarkers found in peripheral bloodhas been validated as a diagnostic test forendometriosis [48, 51].

GLYCOSYLATION-ASSOCIATEDBIOMARKERS

Cancer disease markers of disturbed glycosyla-tion lead to production of tumor-associatedglycans and glycoproteins. These molecules areconsistently secreted or ejected together with acell membrane into the bloodstream and,thereby, can serve as tumor markers. Anincrease in glycosylation in tumor cells is star-ted by means of excessive expression of glyco-proteins that contain certain specific glycans,increase or decrease in nucleotide sugarsdonors, and disturbances in glycosyltransferaseand the glycosidase enzymes expression. Thesemarkers of glycosylation applied to the detec-tion and monitoring of tumors include CA125,CA19-9, carcinoembryonic antigen (CEA),prostate-specific antigen (PSA), and alfa-feto-protein (AFP). Owing to their specific affinity forcertain parts of sugars, lectins are useful forstudying development and identification ofthese tumor-associated glycans and glycopro-teins in clinical practice. Accordingly, variousenzyme-binding lectin assays (ELLA) weredeveloped for diagnostics, monitoring, andprognosis. As glycosylation changes occur inthe early stages of tumors, the tumor-associatedlectin-based glycosylation markers become aneffective strategy to improve diagnosis andtreatment follow-up [59].

CA125 is an antigen-determined high-molecular weight glycoprotein distinguished bya mouse monoclonal antibody OC125. It con-tains two main antigenic domains classified asA, the OC125 monoclonal antibody bindingdomain, and B, the M11 monoclonal antibodybinding domain. CA125 is expressed by amni-otic and coelomic epithelium during gestation.

It has been found in adults in the structures thatdevelop from coelomic epithelium (mesothelialcells of pleura, pericardium, and peritoneum) intubal, endometrial, and endocervical epithelia.It is intriguing that the superficial epithelium ofnormal ovaries of fetuses and adults do notexpress it, except for cases of inclusion cysts orareas of metaplasia and papillary growths.Serum CA125 was defined initially by means ofhomologous assay based on only the usage ofOS125 monoclonal antibody. This research wasreplaced by a heterologous one, using OS125 asa capturing antibody, and M11 as a detectingantibody. Now, there are different techniquesfor CA125 analysis that are clinically reliableand well correlated [60]. The level 35 IU/ml inserum obtained in 1% of healthy female donorsis often assumed as the normal upper level inclinical practice. However, this level is dis-putable and cannot ideally correspond to allCA125 values. For example, in postmenopausalwomen or in patients after a hysterectomy,CA125 level tends to decrease compared to thegeneral population (20 and 26 IU/ml) [61]. Ingeneral, about 85% of patients with ovarianepithelial tumors have CA125 levels higherthan 35 IU/ml. Levels above 35 IU/ml werefound in 50% of patients with stage I ovariantumor while higher serum CA125 levels werefound in more than 90% of women at moreadvanced stages [60, 61]. CA125 rises in muci-nous, clear cell, and borderline tumors lessoften than in malignant ones. Increase in serumCA125 can be associated with other malignanttumors (pancreas, breast, large intestine, andlung) as well as benign and physiological con-ditions including pregnancy, endometriosis,and periods [62]. The majority of these non-malignant conditions are not found in post-menopausal women, which improves thediagnostic accuracy [4, 60, 63].

Many authors consider CA125 to be anendometriosis marker and believe that CA125concentration C 30 IU/ml is an endometriosisindicator with high precision in women withsymptoms of pain and infertility. Ifendometriosis is suspected, CA125 should beconsidered as a diagnostic test, in which theconcentration CA125 is\ 30 IU/ml.

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Nevertheless, it cannot be assumed to revealendometriosis by itself [4, 63–66].

Another ovarian tumor marker, CA19-9, iselevated in endometriosis and has a comparableor lower sensitivity than CA125 for the detec-tion of endometriosis [51]. A recent studyshowed a significant increase of CA125(p = 0.001), CA19-9 (p = 0.015), and CA15-3(p = 0.017) in endometriosis cases (n = 50) ver-sus controls (n = 35). Receiver operating char-acteristic (ROC) curve analysis showed that thearea under the curve was the highest for CA125(0.938). A significant positive correlation withdisease severity was found for CA19-9 as well[67].

INFLAMMATION-ASSOCIATEDBIOMARKERS

Inflammatory markers have been implicated inthe pathogenesis of endometriosis. Manycytokines have been examined as possiblebiomarkers for endometriosis such as IL-1, IL-6,IL-8, tumor necrosis factor-a (TNFa), monocytechemoattractant protein 1 (MCP-1), and inter-feron-c (INFc) [51]. Endometriosis has beenshown to be associated with impaired T cellfunction. Thus, helper T cells produce cytokineIL-4 that is significantly upregulated inendometriotic lesions and intensifiesendometriotic cell proliferation [68]. Peritonealfluid of women with endometriosis contains anumber of Th17 cells as in the ectopic endo-metrium. IL-17 stimulates IL-8 and COX-2expression and enhances proliferation andmigration of endometriotic cells [69]. MCP-1 isa member of the small inducible gene familythat plays a role in the recruitment of mono-cytes to injury and inflammation sites [70].MCP-1 level is elevated in the peritoneal fluidand serum of women with endometriosis, inparticular in early stages [71].

BLOOD/CYCLE-ASSOCIATEDFACTORS

Iron is an essential element for many processescarried out within the cell. Abnormal iron

regulation due to iron overload is destructive[72]. A large majority of iron (i.e., 3–5 g) inhumans is contained in hemoglobin and theremainder is stored in protein complex ferritinin hepatocytes and macrophages [73]. In thecase of iron deficiency, iron can be releasedfrom ferritin by ferritinophagy, which is anautophagic process [74]. Iron supports impor-tant intracellular processes such as oxygentransport, metabolism, and DNA synthesis [75].

Retrograde menstruation is one of thepathogenetic theories of endometriosis devel-opment and subsequent transformation to raretypes of ovarian cancers including endometri-oid and clear cell ovarian carcinomas [76]. Thepelvic cavity may have an elevated iron leveldue to retrograde menstruation, which maypromote desquamated endometrial cell survivaland implantation at ectopic sites [77]. Irondeposits (e.g., hemosiderin) have been revealedin endometriotic ovarian lesions as well as inthe fallopian tubes of patients diagnosed withserous epithelial ovarian carcinomas [78, 79].

Follicular fluid is considered a potentialcontributor to ovarian cancer pathogenesis [80].It is well known that follicular fluid containsactive hormones such as estradiol [80], reactiveoxygen species, and transferrin [81]. Increasedlevels of iron and DNA adducts were founded infollicular fluid of patients with endometriosiscompared to infertile controls [82, 83]. Iron andtranscript levels of ferritin and transferrinreceptors were increased in follicles settlednearby to an endometrioid ovarian cyst affect-ing oocyte maturation [84]. Neverthelessanother study did not find any differences iniron or ferritin contents [85]. Small increases incell proliferation and in IL-8 cytokine levels areobserved after contact with fallopian epithe-lium and follicular fluids [86]. Follicular fluidscontaining high levels of reactive oxygen spe-cies are capable of inducing early-onset B celllymphoma in mammary fat pads in mice lack-ing tumor protein p53 [87]. Some experimentsshow that fimbrial epithelial cells exposed toincreasing doses of iron revealed an increase incellular proliferation and changes in p53,mitogen-activated protein kinase (MAPK), ser-ine-threonine protein kinase (AKT), c-Myc pro-teins, and reactive oxygen species

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(0.05–100 mM) [88]. These oxidative stress-in-duced events mediated by iron were opposed byvitamin D3 [89]. It is notable that a clear cellovarian cancer gene signature was inducedupon iron exposure in immortalized ovariansurface epithelial cells that support the transi-tion from precursor cells to cancer [90].

GENES INVOLVEDIN ENDOMETRIOSISDEVELOPMENT

The gene network of endometriosis includesgenes of hormones and their receptors, tumorsuppressors, the detoxification systems, cytoki-nes, and their receptors, embryonic develop-ment and cell proliferation, and others,involved in more than 30 metabolic ways(Fig. 1) [91]. Epigenetic factors (e.g., level ofDNA methylation, modification of histoneproteins, microRNA) are important in thedevelopment of endometriosis [92].

Somatic mutations and other genomic aber-rations have been found in endometriosis thathave been implicated in the development ofcancer. Mutations in Kirsten rat sarcoma (KRAS)[93, 94], tumor protein p53 gene (TP53)[95, 96], phosphatidylinositol-4,5-bisphosphate3-kinase (PIK3CA) [97, 98], phosphatase andtensin homolog (PTEN) [99], and AT-richinteractive domain-containing protein 1A(ARID1A) gene regions [100, 101] have beendescribed. Microsatellite instability [102], loss ofexpression of mismatch repair enzymes [103],and tissue-specific gene copy-number changes[104, 105] may also be seen in endometriosislesions. Loss of heterozygosity resulting in PTENloss may be an early driver event in the genesisof endometriosis-associated ovarian carcinomasarising from endometriosis [99, 106]. Over thelast 7 years, sequencing and immunohisto-chemical studies have revealed that mutationsfound in endometriosis-associated cancers arefound in adjacent endometriosis [107]. Thesesequencing studies clearly demonstrate a clonalrelationship between benign and malignantcounterparts confirming that the cancers havearisen from the endometriotic lesions[100, 101, 108, 109].

KRAS

RAS proteins include H-, K- and N-RAS, closelyrelated members that activate a wide array ofdownstream signaling pathways with a multi-tude of effector proteins, including Raf/ERK andPI3K/AKT [110]. Extracellular signal-regulatedkinases (ERK1/2) has several substrates, includ-ing epidermal growth factor (EGF) and estrogenreceptors (ERs) [110]. They function as intra-cellular switches in signal transduction cascadesthat regulate proliferation, apoptosis, and dif-ferentiation [111].

Oncogenic KRAS is encoded by the KRAS-2gene [112], is downstream of epidermal growthfactor receptor (EGFR), and is an essentialcomponent of the EGFR signaling cascade[113]. It is frequently mutated in variousmalignancies, such as colorectal cancer (ca.40%), lung cancer (ca. 25%), and pancreaticcancer (ca. 90%) [114, 115]. EGFR inhibitorsbecome ineffective as a result of the impact ofKRAS mutation on the EGFR pathway [116].KRAS also can be activated by angiotensin II[117], endothelin-1 [118], TGFb1 [119], platelet-derived growth factor (PDGF) [120], EGF[121], and thrombin [122]. These genes/mole-cules or their receptors (e.g., angiotensin IIreceptors AT-1 and AT-2) are all reported to beoverexpressed/elevated in endometriosis[123–127].

KRAS mutation attracted much attentionafter the report that the activation of a KRASallele resulted in peritoneal endometriosis inmice [128]. In this model the onset ofendometriosis appeared to be quite late (ca.8 months after conditional induction of KRAS)[128], which raises the question as to whetherthis mouse model of endometriosis truly reca-pitulates the human counterpart. Indeed, KRAS-activating mutation is reported to be rare inendometriomas [90], even though elevatedKRAS expression in eutopic endometrium inwomen with endometriosis has been reported[129–131]. The prolonged latency period ininducing endometriosis suggests that the KRASmutation alone may not be sufficient to induceendometriosis. The mouse model ofendometriosis also shows that the growth of

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lesions is ER-dependent since estrogen antago-nism suppresses the lesion growth [132]. Con-sistent with this, the lesions exhibit fibrosis asseen by marked collagen deposition [132].

KRAS mutations have been identified in 12(29%) of 42 endometriosis-associated ovarianlow-grade endometrioid adenocarcinomas[133]. Inactivating mutation has recently beenreported in deep endometriosis lesions [93].Additionally, membrane-type matrix metallo-proteinase (MT1-MMP) expression has beenconsistently documented to be elevated inectopic endometrium [134] and in peritonealfluid [135] and eutopic endometrium[136] from women with endometriosis. Thus,KRAS mutation is possible in endometriosis, butit seems to be rarely activated [107].

ARID1A

ARID1A is the tumor suppressor encoding pro-tein BAF250 of the switch/sucrose non-fer-mentable (SWI-SNF-A) complex participating inchromatin remodeling. The somatic inactivat-ing mutation of a gene of ARID1A and loss ofexpression of BAF250a is an early disturbance atendometrioid carcinoma of ovaries that belongsto a group endometriosis-associated carcinomas[137]. About 42–61% of clear cell carcinomasand 21–33% of endometrioid carcinomas showloss of the ARID1A gene protein expression(BAF250a) on immunohistochemistry (IHC)[101, 108, 138]. ARID1A regulates proliferationand genomic stability as a tumor suppressorgene; therefore, it was thought that it may playa role in the transformation of endometriosis tocancer [139]. Similar mutations in PIK3CA weredetected between endometriosis and clear celltumors, occurring in early progression mecha-nisms in other cancer types [100]. Multifocalbenign endometriotic lesions are related clon-ally, and clear cell carcinomas arising in thesepatients progress from endometriotic lesionsthat may already carry sufficient cancer-associ-ated mutations to be considered neoplasmsthemselves but with low malignant potential[100].

Studies examining BAF250a expression byIHC have shown that in just over half of the

reported cases of endometriosis-associatedtumors, loss of BAF250a expression is seen in67–80% in areas of contiguous endometriosis oratypical endometriosis. A loss of BAF250a pro-tein expression seems to be an early molecularevent in the development of BAF250a-negativeendometriosis-associated tumors [108, 109,140].

Interestingly, ARID1A mutations are notsufficient on their own to cause cancer [141]. Animportant study recently reported that 65% ofcancer-causing genomic aberrations are randomDNA repair abnormalities [142]. Taking thisinformation into context, one can concludethat BAF250a loss in endometriosis could rep-resent endometriosis-associated tumor-precur-sor lesions; however, ARID1A mutations areneither a necessary driver for mutation nor asufficient determinant of the malignant phe-notype. The presence of mutations inendometriosis may be a sign of broader genomicdisruption leading to the development ofendometriosis-associated tumors. Studies havebeen done comparing patient outcomes in casesof endometriosis-associated tumors based onthe presence or absence of BAF250a expression:the presence or absence of an endometriosisprecursor lesion in endometriosis-associatedtumors has not been associated with a changein overall disease outcome [143].

PI3K/AKT PATHWAY

Presence of classical oncogene mutations inovarian endometriosis was revealed initially in2018: a combination of mutations of KRAS andARID1A was identified in some patients that ledto the preliminary conclusion thatendometriosis is a precancerous disease [144]. Itwas discovered that activation of PI3K/AKTleads to changes in the family of proteins fork-head box protein O1 (FOXO1) influencingthrough the insulin-like growth factor-bindingprotein 1 (IGFBP1) and developing resistance toprogesterone in endometriosis. High levels ofAKT and nuclear proteins of FOXO1 and IGFBP1were evident in ectopic atypical endometrialfoci. These data may demonstrate that the PI3K/AKT pathway participates in the processes of

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hormone resistance in endometriosis. Thisactivating mutation is identified in more than40% of observations in endometriosis-associ-ated ovarian cancer [145]. PIK3CA exons 9 and20 were sequenced in 23 samples of clear cellovarian cancer and activating mutations werefound in 43% of samples—H1047R. The samemutation was revealed in atypical endometrio-sis in 90% of observations. In this regard, theassumption was made that such mutations ariseearly enough that carrying out such analyses foroncological prevention is advantageous ininfiltrative forms of endometriosis [145].

PTEN, or phosphatase and tensin homologdeleted on chromosome 10, is a tumor sup-pressor through regulation of proliferation andsurvival [146]. It is the second most frequentlymutated gene in human cancers after TP53, butthe mutation spectrums of PTEN and TP53 aredifferent [146]. PTEN inhibits PI3K/AKT signal-ing by converting phosphatidylinositol (3,4,5)-trisphosphate (PIP3) to phosphatidylinositol4,5-bisphosphate (PIP2) [146]. Of particularinterest to endometriosis is its ability to main-tain genomic stability [147–149]. Inendometriosis, the first attempt to evaluatePTEN mutation yielded negative results [150],but another study reported a mutation fre-quency of 21% (7/34) in endometriomas [99].One study found no mutation at PTEN (0/23)[94], but the negative finding may beattributable to a lack of adequate statisticalpower due to a small sample size. PTEN loss andloss of heterozygosity have been reported inendometriosis malignant transformation[151–153]. Conditional deletion of PTEN isfound to induce endometriosis in mice [128].Reduced PTEN expression has been reported inendometriosis [153–155]. 17b-Estradiol pro-motes cell proliferation through activatingPI3K/AKT and MAPK/ERK signaling pathwaysvia an NF-jB/PTEN-dependent pathway inendometriotic epithelial cells [153]. IL-8 is alsoreported to enhance proliferation, reduceapoptosis in endometrial stromal cells throughthe upregulation of survivin and B cell lym-phoma 2 (Bcl-2), inhibition of PTEN, and acti-vation of AKT [156]. Consistently, AKTactivation has been shown to promote theestablishment of endometriosis [157]. PTEN

overexpression suppresses cell proliferation, butenhances apoptosis and G1 cell cycle arrest inendometriotic epithelial cells [158].

MTOR SIGNALING PATHWAY

The mechanistic target of rapamycin (mTOR) isa highly conserved, atypical serine/threonine-protein kinase. It interacts with other proteinsto form two multi-protein complexes: themTOR complex mTORC1 and mTORC2. Bothcomplexes contain target of rapamycin com-plex subunit LST8 (mLST8/GbL) and DEPdomain-containing mTOR-interacting protein(DEPTOR). mTORC1 also contains regulatory-associated protein of mTOR (Raptor) and pro-line-rich Akt substrate of 40 kDa (PRAS40) pro-teins, wherein Raptor interacts with the targetof rapamycin signaling (TOS) motifs of mTORsubstrates in a rapamycin-sensitive manner toactivate this complex. mTORC2 also containsrapamycin-insensitive companion of mam-malian target of rapamycin (Rictor), target ofrapamycin complex 2 subunit MAPKAP1(mSIN1), and protein observed with rictor 1 and2 (Protor1/2), and the functional activity ofmTORC2 is dependent on Rictor and mSIN1.mTOR signaling can be activated by upstreamsignals including growth factors (e.g., insulin,insulin-like growth factor 1, IGF1), cellularstress, metabolism/energy state (e.g., O2 andATP/ADP), amino acid nutrients (e.g., leucineand arginine), and neurotransmitters (e.g.,neuropeptides and glutamate). mTOR signalingcontrols several fundamental biological pro-cesses including translation and turnover ofproteins, lipid and glucose metabolism, cellulargrowth, proliferation, survival, autophagy,cytoskeleton organization, etc. Aberrant mTORsignaling has been linked to the pathophysiol-ogy of diseases like cancer, cardiovascular dis-orders, and diabetes.

HOX AND WNT GENES

Homeobox (Hox) genes plays an important rolein mammal embryogenesis and provide theskeletal development, especially the head–tail

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axis. The Hox genes control the differentiationof cells, the segmental embryonic development,involved in malformations of the urogenitaltract and their inactivation leads to completeabsence of Mullerian structures [159, 160].Thus, its absence leads to embryonal death andmultiple organ system abnormalities. The clus-ters of Hox genes during development are sub-ject to transcriptional control by cofactors suchas retinoic acid (RA) [161], FGF [162], and thegenes of the Wnt pathway [163]. Organogenesisof the female genital tract is regulated byhomeobox transcription factors [164]: HoxA-9,HoxA-10, HoxA-11, and HoxA-13 [165]. Thesetranscriptional factors interact with bone mor-phogenetic protein (BMP)-4, Wnt7a, b3-inte-grin, EGFB, and empty spiracles homeobox 2(EMX2) gene and provide normal structures inthe female genital tract. Wnt4 gene is animportant factor in Mullerian duct formation[166]. Its malfunction can lead to anomalies offemale genital organs and in endometrial glan-dular and stromal disintegration. Wnt7 is con-nected to HoxA-10 and HoxA-11 genes asmentioned above, and Wnt5 provides the dis-tribution along the head–tail axis [167]. Wnt5aand Wnt7a are essential for normal endometrialglandular formation [168]. Three types of sig-naling pathways in female genital tractorganogenesis were discovered: Wnt/b-catenin,Wnt/JNK (c-Jun N-terminal kinases), and Wnt/Ca2?. Estrogen and its derivates may interferewith Wnt expression and/or b-catenin targetgenes with an alteration of the female genitalorgan development, and this fact is importantin endometriosis formation [169]. The signalingpathway of Wnt genes and Wnt/b-catenin con-trol different types of stem cells and can provideself-renewal of endometrial stem cells that canlead to future endometriotic foci growth[170, 171].

ATYPICAL ENDOMETRIOSIS

Endometriosis may be divided into peritonealendometriosis, ovarian endometriosis orendometrioma, and deeply invasiveendometriosis. Nowadays, increasing numbersof researchers are inclined to suggest that

adenomyosis and endometriosis are separatediseases because of revealed essential morpho-logic, pathogenetic, molecular, and biologicalfeatures [18]. Results of such research and clin-ical observations lead to differential approachesto clarify the pathogenesis of different clinicaland morphological variants of endometriosisand to study the major pathogenetic factors(genetic, epigenetic, medical, and social)[18, 65]. The existence of different clinical andmorphological forms of endometriosis,depending on prevalence of the active andinactive endometrioid foci, is disputable [18]. Ithas been established that endometrioid hetero-topic foci in ovaries are present in two mor-phologic variants simultaneously: one withendometrioid glands and the other with stro-mal cell growth and active neoangiogenesis.Endometrioid glands and stromal cell growthcause considerable remodeling of an ovary withexpressed fibrosis that is followed by suppres-sion of folliculogenesis accompanied by adecrease in fertility [18].

The atypical form of endometriosis is char-acterized by cellular atypia and excessive pro-liferation; however, it is difficult to establish thediagnosis of atypical endometriosis on the basisof only a morphological evaluation. Malignanttransformation of endometriosis is a rare eventthat occurs approximately in 0.7–2.5% of casesand usually involves ovaries (Fig. 2). It wasdescribed that women with endometriosis have2–3 times higher risk of developing ofendometrioid and clear cell ovarian tumors[172]. Morphological research revealed thecontinuous process consisting of consecutivestages of change from normal epithelium in anendometrioid cyst to atypical endometriosisand, subsequently, to invasive carcinoma [173].Epithelium of endometrioid cyst walls has syn-cytial papillary changes (39.7%), metaplasiawith hobnail cells (15.4%), atypia of an epithe-lium with fluctuation of Ki-67 and Bcl-2expression, with a low p53 expression in 41% ofcases [137]. Hepatocyte nuclear factor 1 home-obox B (HNF-1b) expression in the nucleiwithout atypia occurs in 56% of cases, and inthe nuclei with atypia in 94%, and is found onlyin clear cell adenocarcinomas [137]. Thehyperexpression of HNF-1b allowed the

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adaptative nature and histogenetic connectionbetween ovarian endometriosis and clear cellovarian tumors.

The histological classification of ovariantumors by the World Health Organization(WHO) is based on histogenetic principles. Thisclassification categorizes ovarian tumors withregard to their derivation from epithelial cells,germ cells, and mesenchyme (the stroma andthe sex cord). Epithelial ovarian tumors, whichare the majority of malignant ovarian tumors,are further grouped into histological types asfollows: serous, mucinous, seromucinous,endometrioid, clear cell, transitional cell tumors(Brenner tumors), carcinosarcoma, mixedepithelial tumor, undifferentiated carcinoma,and others [174]. Clear cell and endometrioidcarcinomas are highly associated withendometriosis [175]. Endometriosis-associatedovary carcinomas develop from atypicalendometriosis and have a special profile ofmolecular disturbances, such as mutations ofgenes of ARID1A, PTEN, and CTNNB1 (cateninbeta 1). Endometriosis-associated cancersinclude clear cell and endometrioid ovariancarcinoma. A history of endometriosis has longbeen considered to be a risk factor for laterdevelopment of these malignancies. Recentmolecular genetic evidence has providedunequivocal evidence that these lesions areprecursors for endometriosis-associated cancers[176].

Atypical endometriosis and ovarianendometriosis-associated tumors have commonmolecular damages, such as mutations of PTEN,ARID1A, and hepatocyte nuclear factor-1b(HNF-1b) upregulation. Moreover, mutations ofARID1A were noted in clear cell tumors andatypical endometriosis, but not in the remoteendometrioid foci. Loss of BAF250a proteinexpression assumes the existence of ARID1Amutation and is a useful early marker ofendometriosis malignant transformation[174, 175]. Besides, CTNNB1 mutations in16–53.3%, PTEN mutations in 14–20%, andARID1A mutations in 30–55% of cases werefound in ovarian endometrioid adenocarci-noma. PIK3CA mutations in 20–40% andARID1Amutations in 46–57% of cases are foundin ovarian clear cell carcinoma [174]. While

estrogen and progesterone receptors practicallyalways are absent, HNF-1b often shows hyper-expression in this histological type [177, 178].

The developmental mechanism ofendometriosis-associated ovarian cancersremains unknown. They can evolve via molec-ular transformation of endometriosis or be dueto endometriotic tumor microenvironmentactivity. Additionally, the fact that the presenceof endometriosis improves prognosis is unde-fined, but likely depends on the endometrioticmicroenvironment. The unique microenviron-ment in endometriosis lesions contains epithe-lial, stromal, and immune cells, which adapt tosurvive in hypoxic conditions with high levelsof iron, estrogen, and inflammatory cytokinesand chemokines. Investigation of the uniquemolecular features of the endometrioticmicroenvironment is important in precisiontherapies or prevention design, but the rarity ofwell-characterized clinical samples and thelimited model systems are challenging obstacles[179]. According to the reviewed studies, thereis a reasonable research hypothesis that differ-ent types of endometriosis or endometriosis-associated tumors [180] are different pheno-types of the same genetic changes [181].

PRINCIPLES BEHIND TREATMENT

The clinical diagnosis of endometriosis may bechallenging because signs and symptoms mayoverlap with another gynecologic pathologies,even with cancer. The situation is aggravated bya lack of reliable diagnostic serum biomarkers[182]. Elevated levels of the biomarker CA125are not specific and can indicate the presence ofendometriosis, ovarian cancers, or inflamma-tion [183]. Levels of the serum biomarker HE4(human epididymis protein 4) is considered tobe promising in distinguishing endometriosisfrom ovarian cancers, but its role is controver-sial in different conditions [184]. Endometriosisis suspected clinically in the majority of patientson the basis mainly of history and ultrasoundexamination and treated empirically with hor-monal therapy (e.g., estrogen–progestin con-traceptives or progestin-only therapies) withoutsurgery [185]. Therefore, on the basis of the

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current situation, there is a need for a reliablediagnostic serum biomarker for clinically diag-nosing endometriosis [186].

Surgical treatment with intraoperative frozensection analysis and subsequent histologicalexamination of excised tissue remains the goldstandard for diagnosis of any type of adnexalmass including endometriosis [186, 187]. Sur-gery generally is indicated for patients who areresistant to medical therapy or who are plan-ning pregnancy. Laparoscopy is the typicalapproach [53, 188–191]. Robotic single-sitesurgery and single-port laparoendoscopic sur-gery can be used for the treatment of advanced-stage endometriosis without any differences inoutcome [192].

One of the most common complications ofendometriosis surgery is diminished ovarianreserve with subsequent iatrogenic infertility[193, 194]. There are several surgical techniquesto treat ovarian endometrioid cysts: ultrasound-guided aspiration, excision, and coagulation[195].

Ultrasound (US)-guided aspiration may bethe treatment of choice in the case of ovarianendometriomas, but high recurrence rates havebeen reported (60–90%) [196]. Some authorshave proposed sclerotherapy to reduce therecurrence rate, but the possibility of spillage ofthe endometriotic fluid may lead to adhesiondevelopment [197]. Since the endometrioticfluid consists mainly of clotted blood, US-gui-ded aspiration may increase the risk of bacterialinfection and ovarian abscess formation withfurther oophorectomy. US-guided aspirationand sclerotherapy should be performed inpatients without indications for surgery,patients with increased anesthetic risk, orwomen with severe adhesions [195].

Laparoscopic drainage and coagulation maybe another surgical technique but there is anincreased recurrence rate of endometrioma andproblems of future conceiving in comparisonwith excision [198]. Laparoscopic cystectomyremains the optimal therapy for ovarianendometriomas. Most of the complicationsrelated to ovarian reserve occur during the cys-tectomy techniques, so the question is in whomand when cystectomy should be performed[195].

Medical treatment can include gonado-tropin-releasing hormone (GnRH) agonists insevere cases. Other potential non-surgicaltreatments incorporate hormone receptor(estrogen or progesterone) modulators, immunemodulators, aromatase inhibitors, and anti-an-giogenic drugs [199–201].

Guidelines from the American Society forReproductive Medicine [193, 194] and theEuropean Society of Human Reproduction andEmbryology are focused on pain and infertilitytreatment, as these are possible complicationsof endometriosis. The treatment options incases of pelvic pain include a trial of nons-teroidal anti-inflammatory drugs and hormonaltherapy (combined oral contraceptives). Allhormonal medicines, which encompass com-bined oral contraceptives, progestins, andGnRH agonists, possess similar effectiveness,but their side effects and costs are different.Estrogen–progestin add-back therapy is recom-mended in the case of GnRH agonist prescrip-tion. GnRH agonists are not recommended foradolescents as a result of their effects on bone.The levonorgestrel intrauterine system is effec-tive in some patients. Superovulation withintrauterine insemination show reliable resultsin the case of infertility. Ovarian suppression isnot effective in promoting spontaneous preg-nancy. The use of a GnRH agonist for 3–-6 months is recommended before in vitrofertilization and surgical ablation ofendometriosis in stage I or II disease.

There is a novel treatment option withGnRH antagonists in the case of severe painassociated with endometriosis. The mechanismof action competitively inhibits GnRH receptorsin the pituitary gland and leads to a reductionin circulating gonadotropins and estradiol. Thismechanism is different from that of GnRHagonists which desensitize GnRH receptors inthe pituitary gland and cause depletion ofpituitary gonadotropins that leads to completesuppression of estradiol to levels that areequivalent to bilateral oophorectomy condi-tions [202].

There are some prospective treatmentmethods based on signaling molecules andpathways of endometriosis [13]. JAK/STAT(Janus kinases (JAKs), signal transducer and

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activator of transcription proteins (STAT)) sig-naling pathway plays an important role inendometriosis. STAT activation provides effectsof PDGF, FGF, IL-6, hepatocyte growth factor(HGF), and VEGF. The JAK/STAT pathway is atarget for novel therapeutics because it is rela-tively simple mechanistically, providing a directtranslation of extracellular signals into a tran-scriptional response. Among the small moleculeinhibitors of this pathway are leflunomide (aJAK inhibitor) and atiprimod (a STAT3 inhi-bitor) [13]. In the TGFb/SMAD signaling path-way, the SMAD proteins are the only family oftranscription factors known to propagate TGFbsignals. Lerdelimumab (a recombinant humanIgG4 targeting TGFb2) and metelimumab (ahuman monoclonal IgG4 against TGFb1) caninhibit TGFb signaling [13]. The effects of EGFand TNFa are mediated via the MEK (mitogen-activated protein kinase kinase) pathway.Selumetinib and other MEK/ERK inhibitors arebeing primarily developed and studied for can-cer treatment. However, the role of the MEK/ERK pathway in endometriosis suggests thatthese therapeutic strategies may lend them-selves equally well to this disease [13].

To date, none of the offered strategies for thetreatment of endometriosis leads to completetreatment and avoidance of disease recurrence.Traditional surgical treatment does not alwayspromote complete recovery of ovarian func-tions and does not exclude endometriosisrecurrence. Besides, there is a risk of transfor-mation of endometrioid heterotopic foci intoan atypical form with associated developmentof endometriosis-associated tumors. At thesame time, the molecular mechanisms of suchtransformation remain unclear, complicatingprevention and early diagnosis. The applicationof molecular science with rapidly advancingknowledge of neoplasms and modern tech-nologies creates an opportunity to identify theorigin of endometriosis, especially amongpatients who are at risk of developing cancer. Toseparate patients at risk of endometriosis-asso-ciated cancer from those with benign disease,the application of highly sensitive and specificnovel molecular biomarkers should be explored.A combination of epidemiological,

pathological, and molecular risk stratificationwill be required [203].

CONCLUSIONS

Endometriosis is a complex disease, and its ori-gin remains unclear. It creates certain healthand social problems that warrant intensive sci-entific research to manage this disorder. Atpresent, endometriosis is considered likely aprecancerous lesion, having much in commonin genetic abnormalities. In addition, itsbehavior and association with certain ovariancancers are categorized as endometriosis-asso-ciated ovarian cancers. Many reports clearlyreveal that not all types and not all cases ofendometriosis transform to malignancy. Nev-ertheless, there is still a risk that reasonablycannot be ignored. This perspective demandsanother clinical approach to endometriosismanagement.

Patients suffering from endometriosis maybe divided into several groups according tomalignancy risk. There are schemes and princi-ples to establish categories (i.e., ROMA (risk ofovarian malignancy algorithm), RMI (risk ofmalignancy index), IOTA (International Ovar-ian Tumor Analysis) simple rules) that are basedon serum biomarkers, body mass index (BMI),ultrasound findings etc. A great effort is beingmade to find a non-invasive test to properly andcomprehensively diagnose endometriosis.Many biomarkers have been investigated thatare based on molecular pathways from cell sur-face and nuclear macromolecules. However,results are contradictory in that some serumbiomarkers are elevated in the case ofendometriosis while subsequent histologicalexamination shows benign-looking endometri-otic glands and stroma. It is justifiable to con-jecture that there is a developmental time lagbetween cell atypia and tissue atypical changes.Additional reports have documented subtlehistological changes in endometriosis epithelialcells that precede atypical transformation butwithout any correlation between atypicalchanges and biomarker investigations. More-over, the question regarding the malignanttransformation in endometriomas compared to

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other types of endometriosis remains unan-swered. In conclusion, it should be noted thatthe pathogenesis of endometriosis, its origin,and malignant transformation are not a sepa-rate monolithic disease process, but rather agroup of converging processes that make it evenmore challenging for future investigation.

ACKNOWLEDGEMENTS

Funding. The work was financially sup-ported by the RFBR under scientific project No.18-33-20209 and completed as part of the statetask 0090_2019_0006. The study partially alsosupported by the Russian Academic Excellenceproject ‘‘5-100’’ for the Sechenov University,Moscow, Russia. No Rapid Service and OpenAccess Fees were received by the journal for thepublication of this article.

Authorship. All named authors meet theInternational Committee of Medical JournalEditors (ICMJE) criteria for authorship for thisarticle, take responsibility for the integrity ofthe work as a whole, and have given theirapproval for this version to be published.

Disclosures. Liudmila M. Mikhaleva, Alek-sandr I. Davydov, Olga I. Patsap, Elizaveta V.Mikhaylenko, Vladimir N. Nikolenko, MargaritaE. Neganova, Sergey G. Klochkov, Siva G.Somasundaram, Cecil E. Kirkland and Gjum-rakch Aliev have nothing to disclose.

Compliance with Ethics Guidelines. Thisarticle is based on previously conducted studiesand does not contain any studies with humanparticipants or animals performed by any of theauthors.

Data Availability. All data generated oranalyzed during this study are included in thispublished article/as supplementary informationfiles.

Open Access. This article is licensed under aCreative Commons Attribution-NonCommer-cial 4.0 International License, which permits

any non-commercial use, sharing, adaptation,distribution and reproduction in any mediumor format, as long as you give appropriate creditto the original author(s) and the source, providea link to the Creative Commons licence, andindicate if changes were made. The images orother third party material in this article areincluded in the article’s Creative Commonslicence, unless indicated otherwise in a creditline to the material. If material is not includedin the article’s Creative Commons licence andyour intended use is not permitted by statutoryregulation or exceeds the permitted use, youwill need to obtain permission directly from thecopyright holder. To view a copy of this licence,visit http://creativecommons.org/licenses/by-nc/4.0/.

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